1. Field of the Invention
The present invention relates to a fuel cell which uses as a fuel such a reducing agent as pure hydrogen or reform hydrogen obtained from methanol or a fossil fuel and uses air, oxygen or the like as an oxidizing agent. In more particular, it relates to a gasket used for a polymer electrolyte fuel cell.
2. Description of Related Art
It is known that in a polymer electrolyte fuel cell, in cases where for example the cell uses a cation exchange membrane, which is a proton conductor, as the polymer electrolyte and hydrogen and oxygen are introduced thereinto respectively as the fuel and the oxidizing agent, reactions represented by the following formulas (1) and (2) take place. EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (2)
In the negative electrode, hydrogen dissociates into protons and electrons. The proton moves through the cation exchange membrane toward the positive electrode. The electron moves through electroconductive separator plates, cells stacked therewith in series and further an external circuit and reaches the positive electrode, whereby electricity is generated. In the positive electrode, on the other hand, proton which have moved and reached through the cation exchange membrane, electrons which have moved and reached through the external circuit and oxygen introduced from outside react with one another to form water. Since the reaction is accompanied by heat generation, electricity, water and heat are generated from hydrogen and oxygen, as a whole.
A polymer electrolyte fuel cell differs greatly from other fuel cells in that its electrolyte is composed of an ion exchange membrane, which is a solid polymer. The ion exchange membrane used includes, for example, a perfluorocarbonsulfonic acid membrane (such as that sold under the trade name NAFION, mfd. by Du Pont de Nemours, E. I. Co., USA). In order to show a sufficient proton conductivity, the membrane needs to be in a sufficiently hydrated condition. The hydration of the ion exchange membrane may be effected, as described for example in J. Electrochem. Soc., 135 (1988), p. 2209, by passing the reaction gas through a humidifier to introduce water vapor into the cell and thereby to prevent the drying of the ion exchange membrane. Sealing of each cell may be effected, as described for example in J. Power Sources, 29 (1990), p. 367, by a method wherein the area of the ion exchange membrane is made larger than the electrode area and the circumferential part of the ion exchange membrane which is not bonded to the electrode is held by the upper and the lower gaskets between them.
The materials generally used for the gasket include glass fiber fabric coated with polytetrafluoro-ethylene (such as that sold trade name TEFLON, mfd. by Du Pont de Nemours, E. I. Co., USA) and fluororubber. U.S. Pat. No. 4,826,741 discloses the use of silicone rubber and fluororubber.
FIG. 2 shows an exterior view of a common stack-type polymer electrolyte fuel cell. Separator plates 2 formed of a conductive material, such as glassy carbon, and internal cells (not shown in the Figure) whose circumferential parts are held between insulating gaskets 1 are stacked alternately. A copper-made current collecting plate 3 is closely affixed to the outermost separator plate to form a stack as a whole. The stack is put between stainless steel end plates 5 via insulating plates 4 and the two end plates are bound fast with bolts and nuts. In the Figure, numeral 6 indicates a hydrogen inlet, 7 a hydrogen outlet, 8 an oxygen inlet, 9 an oxygen outlet and 10 a water discharge drain.
FIG. 3 shows a sectional view of an internal cell of a common stack-type cell. Electrodes 12 are bonded to both sides of an ion exchange membrane 11 of the center to form an assembly. Grooved separator plates 2 are positioned at the upper and lower sides of the assembly. The ion exchange membrane 11 has a larger area than the electrode 12, and the circumferential part of the membrane is held by gaskets 1 between them to seal each cell and insulate the separator plates from each other. When, as shown in the Figure, a gas path 13 is provided inside the stack according to necessity (that is, in the case of internal manifold type), the gasket serves also to seal the gas path. The separator plate 2 provided with grooves may have various structures; for example, a porous grooved plate is fixed into the groove, or a wire mesh is used in the groove.